DYNAMICS OF BACTERIA AND ORGANIC CARBON IN RIVER SEDIMENTS: IMPORTANCE OF PARTICULATE ORGANIC CARBON

The distribution of organic carbon and bacterial activity were investigated in riparian sediments of an oligotrophic alpine river impoundment (River Enns, Austria). River and interstitial water, and hyporheic sediments were sampled over an annual cycle. Bacterial abundance and activity in the river water correlated with temperature and river discharge but not with the dissolved organic carbon (DOC) concentration. The dynamics of hyporheic bacteria followed the fluctuations in the river water. This coupling, however, decreased with depth of the sediment, and in deeper sediment layers bacterial activity was mainly depending on the supply of particulate organic carbon (POC). Sediment-associated POC concentration as well as abundance and activity of interstitial bacteria decreased by one order of magnitude within the first meter of sediment. Hyporheic bacterial activity was mainly sediment-associated and exceeded the DOC input. Comparison of the hyporheic bacterial carbon demand with the total organic carbon input from the river clearly indicates that POC rather than DOC input is the main source of hyporheic bacterial activity.